I have flown Southwest about 6 times now and some of those flights have had connections. I have NEVER seen anyone with one child under the age of 4 board with just one parent.
Just for the sake of arguement--what happens if your dh is not with you? Would you make your 7 year old board alone because he or she is over 4?
I have seen one parent board with only one child plenty of times while the grandparents/other adult boarded later. Heck, I have even seen flights where SWA didn't announce family boarding because the flight was full.
Of course if I was the only adult on a flight and I was flying with both a 4yr old and a 7 yr old I would then have all of us board together. There's a big difference between that scenario and the scenario you are contemplating, don't you think?